In line with rapid increase in use of fossil fuels, demand for use of alternative energy or clean energy is increasing. Thus, the field of power generation and electricity storage, which use electrochemical reaction, is most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and use thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating voltage, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles, hybrid electric vehicles, and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, a nickel-metal hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively carried out and some lithium secondary batteries are commercially available.
Conventional lithium secondary batteries use graphite as an anode active material and charging and discharging processes are performed while lithium ions of a cathode are repeatedly intercalated into and deintercalated from an anode. Although there are differences in theoretical capacities of batteries according to kinds of electrode active materials, in most cases, charge and discharge capacities are deteriorated as cycles proceed.
Such phenomenon is mainly attributed to non-functioning of active materials due to separation of electrode active material components or separation between an electrode active material and a current collector by change in volume of an electrode occurring as charging and discharging of a battery proceed. In addition, in the intercalation and deintercalation processes, lithium ions intercalated into an anode are unable to be properly deintercalated therefrom and thus an anode active site is reduced and, accordingly, charge and discharge capacities and lifespan characteristics of a battery are deteriorated as cycles proceed.
In particular, to increase discharge capacity, when natural graphite having a theoretical discharge capacity of 372 mAh/g and a material having a high discharge capacity such as silicon, tin, Si—Sn alloy, or the like are used in combination, volume expansion of these materials significantly increases as charging and discharging proceed. Accordingly, an anode material is separated from an electrode material and, consequently, battery capacity is dramatically reduced as cycles are repeated.
Thus, there is an urgent need in the art to study a binder and an electrode material that may have strong adhesive strength so as to prevent separation between electrode active material components or separation between an electrode active material and a current collector when manufacturing an electrode and may have strong physical properties so as to achieve structural stability of an electrode by controlling volume expansion of an electrode active material caused as charging and discharging are repeated and, accordingly, enhance battery performance.
A conventional organic solvent-based binder, i.e., polyvinylidene fluoride (PVdF), does not meet such requirement and thus, recently, a method of using binders prepared by preparing emulsion particles by aqueous polymerization of a water-based binder such as styrene-butadiene rubber (SBR) and mixing the emulsion particles with a thickening agent and the like has been proposed and such binders are commercially available. These binders are eco-friendly and used in a small amount and thus may increase battery capacity.
However, an existing thickening agent such as carboxymethyl cellulose deteriorates battery processability due to difficulty in increasing solids of an electrode slurry while having high viscosity and stability. In addition, the thickening agent undergoes decomposition of a carboxyl group at high temperature and thus swelling in which an electrode assembly is inflated when left at high temperature occurs due to gases generated at high temperature.
Therefore, there is an urgent need to develop a lithium secondary battery including an aqueous binder that enhances overall characteristics of a battery, imparts structural stability to an electrode, and has high adhesive strength.